Q: Are there any other tokamaks in operation around the world and when will fusion power be used in ships and how much of a reduction in size would it take?
A: There are many tokamaks in operation around the world, all contributing to an international effort to realise commercial fusion power. As well as EFDA-JET, there are many other tokamaks in operation in Europe (the UK has its own tokamak, MAST as well as operating JET on behalf of Europe). There are also tokamaks operating in the USA and Japan, and smaller tokamak experiments all over the world (in counties such as Brazil, India, Australia, China, Russia etc.).

There are no plans (as far as I am aware) to use fusion power plants directly in ships - there would be difficulty in making them small enough, given our present level of knowledge. Certainly in terms of other forms of transport, the use of fusion to generate electricity would obviously be directly relevant to the railways and, one could envisage, cars well in the future running on electricity.

Q: What are the minor and major radii of the plasma?
A: The major radius of a tokamak plasma is the radius of the tokamak as a whole (from the centre of the hole down the centre of the device to the the centre of the plasma) and the minor radius is the radius of the plasma itself.

Q: How much more energy will it take a Tokamak over the "breakeven point" to actually produce power due to loss of energy in the processes of electricity production?
A: The fusion energy balance breakeven is achieved when the energy from fusion reactions is larger than the energy required to sustain the plasma. However, in order to produce a net amount of energy (converted as electricity), the power plant must produce significantly more energy than that required to power all plant auxiliary systems (engineering breakeven). The required power amplification from the plasma, in order to achieve overall engineering breakeven, depends on how the plant design is optimised, but it is expected to be of the order of 30.

Q: Would a Tokamak work better in zero/low gravity conditions?
A: Gravity has very little effect on the dynamics and/or stability of the plasma so operation of a tokamak in zero gravity would make little difference.

Q: How does plasma heat if all the particles are travelling in the same direction?
A: Actually, plasma particles are not bound to travel in the same direction. In a gas, particles can move freely in three dimensions. In JET (as well as in any other magnetic confinement facility) a charged particle loses one degree of freedom, i.e. it is free in two dimensions. One is along magnetic field lines (ie close to centre line of the toroid and slightly helical). The second is its rotation around a magnetic field line. In any of the two dimensions, every particle is free - it can gain or lose velocity.

In a magnetically confined plasma the velocity distribution of particles is thermal (Maxwellian) in both dimensions. In other words, plasma particles can fly in different directions (clockwise, counterclockwise around the torus / around the field line) and with different velocities. The Maxwellian distribution of velocities is maintained by mutual collisions of the particles (and, in plasma, by electromagnetic interactions too). There are only two global parameters: the width of the velocity distribution (which corresponds to temperature) and the offset of its maximum from zero. The latter correspond to a collective motion - a wind in gas, or, in JET we call it plasma rotation. Plasma temperature in JET rises above 100 millions deg Celsius.

I hope that now you see that even in the hypothetical case of one dimensional distribution (say, from left to right) there is a measure of temperature - particles can move chaotically from right to left or vice
versa, fast or slow. If their mean velocity is zero, there is no "wind". Temperature is proportional to the mean value of the square of their velocities. When particles stop completely we say temperature is absolute zero (-273.15 deg Celsius). On the other hand, if the temperature is high, they mutually collide with big impulses so that some can fuse.

Sometimes students exaggerate the effect of the plasma electric current which is the source of Joule heating in tokamaks. Indeed, electric current is also a velocity distribution offset, with opposite directions for +ions and for electrons. However, thanks to many collisions in plasma, this offset is small compared to typical thermal velocities.

Q: I would think that ITER would require a substantial amount more power to produce the plasma than JET. If you were to create a smaller reactor you would need less energy to run it, thus allowing for it to run longer which should allow you to study it better. If you make it smaller you can exert more magnetic force to contain and heat the plasma and possibly make it self sustaining. Shouldn't we be looking at smaller tokamaks?
A: It is true that ITER will require more power to heat the plasma than JET as the plasma is bigger and will need to be hotter. However, ITER (as it is hotter and bigger) will produce a much more efficient fusion reaction than JET and will consequently reach an energy gain factor (fusion energy divided by the energy required to heat the plasma) of 10 (JET has only reached 0.7). In addition, electricity is needed to run the magnetic field coils in a tokamak and this is a large amount on JET. On ITER, the electricity requirements of these coils is small as they will be superconducting and will require little voltage to keep them running. The duration of the plasma in ITER will be much longer than JET (5-10 minutes compared to 10s or so in JET) - the restriction on the plasma duration comes from the discharging transformer required to induce the plasma current in a tokamak - this will be much larger on ITER than JET - also, hotter plasmas require less induced voltage to keep them running.

Creating smaller reactors would be very good, but the confinement of plasma particles gets worse as the plasma gets smaller (and good confinement is required for effective fusion). So, the chances of having a fusion powerplant under the bonnet of your car are remote!

Q: Would it be best to devote a majority of resources to the Tokamak projects instead of Z pinch machines or laser type fast igniter approaches? Besides the technical spin-offs that the other approaches contribute, is the Tokamak more efficient design toward power plant production?
A: I cannot really comment on which is the best approach towards a fusion powerplant. I can make a few comments on all these options, although you should realise I am by no means an expert on Z pinches or Laser driven inertial confinement systems:

Z-pinch - this concept is not being studied as intensively as the other two concepts you mentioned and it would be fair to say that the research in this area is significantly further away from realisable fusion power - certainly further away than the tokamak.

Laser driven inertial confinement - There is a significant programme (especially in the US) into laser driven fusion systems (lasers with directly or indirectly imploding a solid capsule of D and T), although it is probably fair to say that the research is a little behind the tokamak. Problems with this approach include instabilities in the collapsing capsule if the radiation incident on it is not perfectly aligned.

Tokamak - this is the basis of the research carried out at Culham (e.g. on JET) and many other labs around the world. Fusion has been observed in JET (albeit less was observed than the energy required to heat the plasma to fusion temperatures) as the plasma was heated and confined (using powerful magnetic fields) sufficiently for this to happen. There are problems with tokamak research (plasma confinement, stability etc.) but the full scale powerplant suggests that the tokamak is probably closer than the other approaches to commercial power production. However, research in Laser driven systems is heavily funded in the US and may well prove an equally feasible system.